The tail-light issue seems to be quite common with the tail-lights Thor uses (among several other RV manufacturers that use them).
I discovered this issue last year and I was able to epoxy the inside areas to prevent any further damage.
Here is a thread where the tail-light issue was discussed. Might shed some light on the issue:
http://www.thorforums.com/forums/f8/...issue-808.html
The problem is - as you suspected, a poor design of the tail lights. Also, the gasket is a joke. I bought new gaskets from the company that makes the tail lights (Command Electronics), but it is only a matter of time before I have issues again.
The only solution as I see it is to silicone the tail lights - which will alleviate the leak problems, but making it difficult to replace a burned out bulb - or replace the tail lights with something better.
Later this year, when the weather permits, I am going to replace the tail-lights with better ones. I bought a pair of Bargman 34-84-009 tail lights which are fairly close to the ones on the coach, but may require some minor trimming to enlarge the cut-out.
The Bargman tail lights are OEM for the better coaches such as Jayco and Coachmen, while the cheaper Command Electronics lights are OEM on Thor, Winnebago, and a few other cheaper coaches.
I would recommend anyone having the Command Electronics tail lights to often check them to ensure they are not leaking. At minimum, epoxying the inside cavity will prevent rot issues, and I would suggest that as well.
So it is not a Thor issue in the sense they don't manufacture the tail lights, but it is a Thor issue in the sense they are using them.